Electrochemical glucose test strips, such as those used in the OneTouch® Ultra® whole blood testing kit, which is available from LifeScan, Inc., are designed to measure the concentration of glucose in a blood sample from patients with diabetes. The measurement of glucose can be based on a physical transformation (i.e., the selective oxidation) of glucose by the enzyme glucose oxidase (GO). The reactions that can occur in a glucose biosensor are summarized below in Equations 1 and 2.Glucose+GO(ox)→Gluconic Acid+GO(red)  Eq. 1GO(red)+2Fe(CN)63−→GO(ox)+2Fe(CN)64−  Eq. 2
As illustrated in Equation 1, glucose is oxidized to gluconic acid by the oxidized form of glucose oxidase (GO(ox)). It should be noted that GO(ox) may also be referred to as an “oxidized enzyme.” During the chemical reaction in Equation 1, the oxidized enzyme GO(ox) is transformed to its reduced state, which is denoted as GO(red) (i.e., “reduced enzyme”). Next, the reduced enzyme GO(red) is re-oxidized back to GO(ox) by reaction with Fe(CN)63− (referred to as either the oxidized mediator or ferricyanide) as illustrated in Equation 2. During the re-generation or transformation of GO(red) back to its oxidized state GO(ox), Fe(CN)63− is reduced to Fe(CN)64− (referred to as either reduced mediator or ferrocyanide).
When the reactions set forth above are conducted with a test voltage applied between two electrodes, a test current can be created by the electrochemical re-oxidation of the reduced mediator at the electrode surface. Thus, since, in an ideal environment, the amount of ferrocyanide created during the chemical reaction described above is directly proportional to the amount of glucose in the sample positioned between the electrodes, the test current generated would be proportional to the glucose content of the sample. A mediator, such as ferricyanide, is a compound that accepts electrons from an enzyme such as glucose oxidase and then donates the electrons to an electrode. As the concentration of glucose in the sample increases, the amount of reduced mediator formed also increases; hence, there is a direct relationship between the test current, resulting from the re-oxidation of reduced mediator, and glucose concentration. In particular, the transfer of electrons across the electrical interface results in the flow of a test current (2 moles of electrons for every mole of glucose that is oxidized). The test current resulting from the introduction of glucose can, therefore, be referred to as a glucose current.
Because it can be very important to know the concentration of glucose in blood, particularly in people with diabetes, glucose meters in the form of episodic glucose meter or continuous glucose monitors have been developed using the principles set forth above to enable the average person to sample and test their blood for determining their glucose concentration at any given time. The glucose current generated is detected by the glucose meter and converted into a glucose concentration reading using an algorithm that relates the test current to a glucose concentration via a simple mathematical formula. In a popular form of the glucose meter, the glucose meters work in conjunction with a biosensor (which is disposable) that may include a sample-receiving chamber and at least two electrodes disposed within the sample-receiving chamber in addition to the enzyme (e.g. glucose oxidase) and the mediator (e.g. ferricyanide). In use, the user pricks their finger or other convenient site to induce bleeding and introduces a blood sample to the sample-receiving chamber, thus starting the chemical reaction set forth above.
For glucose measurements made using electrochemical sensors, such measurements are susceptible to measurement errors arising from the presence of endogenous and exogenous substances in the blood sample (interferent compounds). Such interferent compounds give rise to a measurement error through two mechanisms. Firstly, the interferent compound may be directly oxidised at the electrode surface, giving rise to an error current. Secondly, the interferent compound may react with the mediator, giving rise to an error current.